Conventional nodular cast iron having a ferrite background or ferritic nodular cast iron, such as FCD37 and FCD40 (JIS G5502--Spheroidal Graphite Iron Castings) demonstrates a large elongation and high impact strength but has a low tensile strength and poor low-temperature impact strength. On the other hand, nodular cast iron having a pearlite background or pearlitic nodular cast iron, such as FCD50 and FCD60 (JIS G5502), has a high tensile strength and yield strength but demonstrates a relatively small elongation and low impact strength, particularly at low temperatures.
Component parts made of cast iron for automotive and other industrial uses are now desired to have an even higher toughness and, additionally, as they are sometimes used at as low a temperature as approximately -40 .degree. C., are now required to maintain a high impact strength even at such low temperatures.
To the end of making improvements in this respect, Japanese Patent Publication No. 61-33897 proposes the addition of nickel to the nodular graphite cast iron. However, the impact strength of the nodular cast iron based on this proposal is limited only to 1.7 kgf-m/cm.sup.2 at -15 .degree. C. Furthermore, according to this proposal, the material is required to have a completely ferritic structure. Therefore, a ferritizing process is required to be performed subsequent to the casting process, but, in view of reducing the manufacturing cost, it is more desirable to do away with any such heat treatment subsequent to the casting process and permit the use of the component parts made of nodular cast iron as cast.
Japanese Patent Publication No. 59-17183 filed by one of the assignees (applicants) of the present application discloses a nodular cast iron which contains nickel and can be used as cast without any heat treatment.
U.S. Pat. No. 4,432,793 recommends the use of a substantially large dosage of bismuth in nodular cast iron for obviating the need for heat treatment.